Propulsion systems for marine vessels are typically calibrated to operate within narrow parameters in order to achieve efficient operation. In particular, the hydrodynamic properties of a marine propeller are generally closely matched to the speed and power of an associated motor; the weight, weight distribution and hull resistance of the vessel; and the environment, such as the water temperature, within which the vessel operates.
Many engines for use within marine vessels are electronically controlled to adjust their power output depending on the ambient air and water temperatures within which the vessel is operating. This can make choice of a propeller difficult, as it is important that the propeller be designed such that a minimum speed of revolution is reached when the engine throttle is completely opened, in order to prevent overloading of the engine.
The complexities of propeller design are further exacerbated by the prospect that the speed of rotation of the propeller will vary depending on the extent of sheet cavitation. The amount of cavitation varies considerably according to the speed of the vessel, the density and temperature of the water within which the propeller is working, as well as the hydrodynamic properties of the hull and shaft line. Cavitation can result in excessive vibration, wear and loss of efficiency of a propeller.
Although complex, the hydrodynamic properties of propellers are sufficiently well understood that it is possible to design a propeller to match the known characteristics of a marine vessel and engine. Problems arise, however, when characteristics of a vessel are changed, for instance by the addition of new features such as a fishing tower, or by the relocation of the vessel from a cold water environment to a warm water environment.
Known solutions for this problem range from the replacement of the propeller—which can be a very expensive procedure—to manual bending of the propeller blades. Bending of the blades alters the propeller's hydrodynamic properties in substantially uncontrollable ways, and also introduces stresses which can lead to fatigue cracking and ultimate mechanical failure of the blades.
The present invention seeks to at least partially ameliorate these problems, and to provide a means for altering the hydrodynamic properties of a marine propeller in a controlled manner.